As we all know, the destruct sequence on the original Enterprise required voiceprint and code verification from the top three officers serving at the time.
Isn't that assumed rather than specified?
Could easily be any three officers of Lt. Cmdr and above including the current CO.
In TNG it is specified to require the CO and XO, though one assumes the computer would know if Picard and Riker were not aboard to accept input from whoever is currently acting CO and XO, e.g. Data and Geordi. Losing the ability to blow up your toys rather than let them fall into the wrong hands seems to be a big deal for Starfleet, so I doubt they would write in such a big bug.
Of course, there is always "shoot a phaser at the warp core" - but destroying the ship doesn't extend to suicide, at least not for Janeway... (though of course you could argue given time she would have done so, she had only a split second before the Kazon got to the bridge).